It is well known in the art that the essence of high fiber tensile strength and modulus is the capability of macromolecules, of adequately high molecular weight, to adopt a fully extended (rod-like) conformation and good parallelization (orientation) with respect to the fiber axis. With fairly stiff-chain para-aramids, exemplified by poly(p-phenylenetere-phthalamide) (PPD-T), good orientation is accomplished by virtue of the ability of precursor liquid crystalline spinning solutions to become well oriented on extrusion through a spinneret and prior to coagulation. Fibers thus formed are not capable of significant draw-orientation, although modulus, and sometimes, to a lesser extent, tenacity may be improved by heat-treatment under tension, which is in effect a crystallization process. A second route to oriented aramid fibers of high strength and modulus is by choice of a semi-flexible composition such that, while it cannot form ordered, liquid crystalline solutions, an initially amorphous as-spun fiber may be very highly stretched at elevated temperature to bring the chains into good alignment. These are typically random copolymers which are non-crystalline in the drawn state. Random copolymers based on the corresponding diacid, 3,4'-oxydibenzoic acid (3,4'-ODB) have been disclosed in a general way, e.g., as part of a large number of possibilities for combining certain cited monomers (Japanese Patent Application 78-143726), or as within the description of a generalized formula (H. Sasaki et al., U.S. Pat. No. 4,507,467). Such drawable copolymers generally do not have the capability for substantial crystallization, which is known, as cited earlier, to enhance orientation which is a major prerequisite for maximization of fiber strength. Herein is disclosed copolymer based on 3,4'-oxydibenzoic acid and selected, substituted bibenzoic acids, and p-phenylenediamine, which can be highly drawn to provide large improvements in tensile strength and modulus, and which are substantially crystalline with an unusually high level of orientation. The crystallite regions are novel in that both diacid units are isomorphously incorporated therein, as indicated by a unique copolymer diffraction pattern.
H. W. Schmidt and D. Guo, Makromol. Chem. , vol. 189, p. 2029-2037 (1988) report the use of 2,2'-dimethyl-4,4'-bibenzoic acid units in an aromatic polyester. No mention is made of the use of this compound in an aramid.
U.S. Pat. Nos. 4,384,107 and 4,461,886 disclose aramids containing either stilbinyl and/or biphenylyl units. Among the possible biphenylyl type units are those derived from 2,2'-disubstituted-4,4'-bibenzoic acid, and specifically mentioned substituents are chloro, bromo, nitro and methyl. No mention is made of the use of other units claimed herein in a random copolymer.
U.S. Pat. No. 4,843,141 discloses polyesteramides containing units derived from 2,2'-disubstituted-4,4'-bibenzoic acid, an aromatic aminophenol, and optionally an aromatic diacid, such as terephthalic acid, and also optionally an aromatic hydroxy acid or amino acid. Specifically mentioned substituents include chloro, bromo, nitro and methyl. No mention is made of the use of these bibenzoic acids in aramids (as opposed to polyesteramides).